It is known that an underwater vessel (i.e., a submarine) generates sound as it travels through the water. The sound is generated by a variety of sources, including, but not limited to, sound generated by a submarine propulsion system, sound generated by a submarine propeller, and sound generated by a submarine electrical power generator. It is known that submarine designers attempt to reduce these and other sound sources in order to make a submarine difficult to detect by passive acoustic means, therefore remaining as covert as possible.
Some anti-submarine warfare (ASW) sonar systems attempt to detect the underwater sound generated by a submarine. Some other ASW sonar systems attempt to both detect the sound and also to localize and/or track the submarine. Localization is used to identify a position of the enemy submarine in azimuth, and/or in range, and/or in depth. Conventional systems that localize and track a source of sound require a substantial amount of human assistance.
Some known sonar systems use autonomous portions. For example, sonobuoy systems use one or more sonobuoys, which are autonomous free-floating units, some types of which are capable of providing beamforming processing to generate receive beams. Conventional sonobuoys provide output signals representative of only sound in the water, but, by themselves, are not capable of detecting, localizing, tracking, or classifying targets in the water. Furthermore, in general, the receive beam provided by a sonobuoy are statically steered only to fixed azimuthal or vertical angles.
Sonobuoy systems include not only the autonomous one or more sonobuoys, but also must include another asset, for example, an aircraft, with more advanced processing capabilities to receive the signals from the one or more sonobuoys. It is the other asset, e.g., the aircraft, which is configured to detect, localize, track, and classify targets. Within the aircraft, the detection, localization, tracking, and classification are performed with substantial human assistance.
As with the sonobuoy systems, some shipboard conventional passive sonar systems can beamform, detect, localize, track and classify a submarine. However, these systems are not autonomous and they rely upon a large amount of human assistance. For example, one type of conventional ship-board sonar system may beamform with an acoustic array, for example, a towed array, partially process the beamformed signals, and provide on a computer monitor a visual display of the partially-processed information. The visual display can have a variety of forms. One such form of display is a waterfall frequency domain display. A human operator may visually detect a submarine (i.e., make a detection and a classification) by way of the visual display, for example, by visual recognition of spectral line patterns representative of a narrowband frequency of sound generated by the submarine. The human operator may then assist in localizing and tracking the submarine.
It will be appreciated that identifying (detecting) the relatively quiet submarine from within a large number of relatively loud ships in the same area can be a formidable problem. Furthermore, it will be appreciated that localizing, tracking, and classifying the submarine once detected is also a formidable problem.
Even at relatively short ranges, localization in depth and range is not generally possible by conventional passive sonar systems, even with human assistance. This is because for any receive beam and associated vertical angle that points toward a submarine, the submarine can be positioned at an essentially infinite number of depths and ranges along the vertical beam steering angle.
At longer ranges, localization of the submarine in range and depth is made even more difficult by a variety of factors, including but not limited to, a tendency of the sound generated by the submarine to bend (i.e. refract), primarily in a vertical direction, as the sound propagates through the water. Therefore, the vertical angle of arrival at which the greatest amount of sound arrives at the sonar system, which is related to a particular receive vertical beam angle, does not necessarily point directly toward the submarine.
With human assistance, (i.e., in non-autonomous arrangements) a variety of processing techniques are used by conventional passive sonar systems. For example, some passive sonar systems use narrowband spatial processing. Narrowband matched field processing is a known technique used to localize a submarine in range and in depth. Narrowband processing generally requires a large sonar array, which is not practical for many applications. Narrowband matched field processing also suffers from the effects of the above-described sound refraction.
For another example, with human assistance, some passive sonar systems use broadband processing. Broadband autocorrelation processing is a known technique in which a signal received by a sonar element (i.e., sonar transducer), or a sonar array, is autocorrelated to identify a relative time delay between the sound arriving at the sonar element on a direct sound path and the sound arriving at the sonar element on a surface-reflected sound path. The relative time delay can be used to calculate range and depth. However, the performance of this technique can be greatly degraded at moderate to high sea states (i.e., when the sea surface has high waves) due to scattering of the sound reflected from the surface, which causes the autocorrelation to degrade.
Some sounds in the water tend to be amplitude modulated by the sound field emitted by a vessel's propellers. In particular, broadband sound received by the passive sonar system can be amplitude modulated in a manner related to characteristics of the propeller.
For yet another example, some passive sonar systems use amplitude modulation of the received sound in order to identify characteristics of the propeller, for example, rotation speed and number of propeller blades. With this information, the passive sonar systems, with human assistance, may be able to classify the type of vessel, including, but not limited to, whether the vessel is a surface vessel or a submarine. The processing can be of a type referred to as “detection of envelope modulation on noise.” One conventional type of detection of envelope modulation on noise is conventionally referred to as DEMON.
In addition, some passive sonar systems use adaptive beamforming, wherein receive beams are steered so as to reduce their response in the directions of noise sources in the water, e.g., surface ships, that are not of interest.
It would be desirable to provide an autonomous system that can adaptively beamform, and also detect, localize, track, and classify sound generated by targets in the water, all without human assistance.